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Basics of PLC Programming Industrial Control Systems Fall 2006
Lecture – PLC Programming Basics
MME 486 – Fall 2006
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PLC Architecture
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PLC System
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Processor Memory Organization The memory of a PLC is organized by types. The memory space can be divided into two broad categories: program and data memory. Advanced ladder logic functions allow controllers to perform calculations, make decisions and do other complex tasks. Timers and counters are examples of ladder logic functions. They are more complex than basic inputs contacts and output coils and rely heavily upon data stored in the memory of the PLC. Lecture – PLC Programming Basics
MME 486 – Fall 2006
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Memory Map A memory map can be used to show how memory is organized in a PLC. § Input/output locations § Internal relay and timer/counter locations
Data table
User program
The user program causes the controller to operate in a particular manner
Housekeeping memory
Used to carry out functions needed to make the processor operate (no access by user)
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Program Files The user program will account for most of the memory of a PLC system.
• Program files contain the logic controlling machine operation.
Program file organization for SLC-500 controller. Lecture – PLC Programming Basics
• This logic consists of instructions that are programmed in a ladder logic format. MME 486 – Fall 2006
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Data Files Data file organization for SLC-500 controller.
The data file portion of memory stores input and output status, processor status, the status of various bits and numerical data.
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Data Files Data files are organized by the type of data they contain
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Input Table File Operation Input module Switch Open
Binary 0 stored 0
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Input Table File Operation Input module Processor continually reads current input status and updates input image table file
Switch Closed
Binary 1 stored 1
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Output Table File Operation Output module Output OFF
Processor continually activates or deactivates output status according to output image table file status 0 Status 0
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Output Table File Operation Output module Processor continually activates or deactivates output status according to output image table file status
Output ON 1
Status 1
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Program Scan During each operating cycle, the processor reads all inputs, takes these values, and energizes or de-energizes the outputs according to the user program. This process is known as a scan. I/O scan – records status data of input devices. Energizes output devices that have their associated status bits set to ON (1)
Program scan – instructions are executed sequentially
Because the inputs can change at any time, the PLC must carry on this process continuously. Lecture – PLC Programming Basics
MME 486 – Fall 2006
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Scan Process The scan time indicates how fast the controller can react to changes in inputs. Scan times vary with computer model and program content, and length. If a controller has to react to an input signal that changes states twice during the scan time, it is is possible that the PLC will never be able to detect this change.
Scan time may be a concern in high speed operations
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Scan Process Read inputs
The scan is a a continuous and sequential process Adjusts outputs
Lecture – PLC Programming Basics
Run program
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Data Flow Overview
Input modules
Input data
Input image table file
Output image table file
Examine data
Output data
Output modules
Return results Program
Check/compare/examine specific conditions Lecture – PLC Programming Basics
MME 486 – Fall 2006
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Scan Process Output Module
Input Module Input device
Input file
Output file
I:3/6
O:4/7
Output device
O:4/7
I:3/6 Program When the input is closed, the input module senses a voltage and an ON condition (1) is entered into the input table bit I:3/6
I:3/6
O:4/7
During the program scan the processor sets instructions I:3/6 and O:4/7 to ON (1)
Lecture – PLC Programming Basics
MME 486 – Fall 2006
The processor turns light output O:4/7 ON during the next I/O scan
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Scan Patterns Horizontal Scanning Order The processor examines input and output instructions from the first command, top left in the program, horizontally, rung by rung.
End of ladder
In addition to the program itself, the scan time is also dependent on the clock frequency of the processor! Lecture – PLC Programming Basics
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Scan Patterns Vertical Scanning Order The processor examines input and output instructions from the first command, vertically, column by column and page by page. Pages are executed in sequence.
End of ladder Misunderstanding the way the PLC scans can cause programming bugs! Lecture – PLC Programming Basics
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PLC Programming Languages The term PLC programming language refers to the method by which the user communicates information to the PLC.
The three most common language structures are: ladder diagram language, Ladder diagram language Boolean language, and functional chart. Functional chart Boolean language Lecture – PLC Programming Basics
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Comparing Programming Language PB1
CR1
CR2
SOL
Relay Schematic
LS1 PB1 CR1
CR2
SOL
Equivalent ladder diagram language LS1
Equivalent Boolean language
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Relay-Type Instructions The ladder diagram language is basically a symbolic set of instructions used to create the controller program. These ladder instructions symbols are arranged to obtain the desired control logic.
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Examine If Closed (XIC) Instruction Symbol
Analogous to the normally open relay contact. For this instruction we ask the processor to EXAMINE IF (the contact is) CLOSED (XIC)
Typically represents any input. Can be a switch or pushbutton, a contact from a connected output, or a contact from an internal output. Has a bit-level address which is examined for an ON condition. The status bit will be either 1 (ON) or 0 (OFF). Lecture – PLC Programming Basics
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Examine If Closed (XIC) Instruction
I:012 I:012 04
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Examine If Closed (XIC) Instruction
I:012 I:012 04 If the status bit is 0 (OFF), then the instruction is FALSE. Lecture – PLC Programming Basics
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Examine If Closed (XIC) Instruction
I:012 I:012 04 If the status bit is 1 (ON), then the instruction is TRUE.
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Examine If Open (XIO) Instruction Symbol
Analogous to the normally closed relay contact. For this instruction we ask the processor to EXAMINE IF (the contact is) OPEN (XIO).
Typically represents any input. Can be a switch or pushbutton, a contact from a connected output, or a contact from an internal output. Has a bit-level address which is examined for an OFF condition.
The status bit will be either 1 (ON) or 0 (OFF). Lecture – PLC Programming Basics
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Examine If Open (XIO) Instruction
I:012 I:012 04 If the status bit is 0 (OFF), then the instruction is TRUE.
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Examine If Open (XIO) Instruction
I:012 I:012 04 If the status bit is 1 (ON), then the instruction is FALSE.
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Output Energize (OTE) Instruction Symbol
Analogous to the relay coil. The processor makes this instruction true (analogous to energizing a coil) when there is path of true XIC and XIO instructions in the rung.
Typically represents any output that is controlled by some combination of input logic. Can be a connected device or an internal output (internal relay). If any left-to-right path of input conditions is TRUE, the output is energized (turned ON). Lecture – PLC Programming Basics
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Output Energize (OTE) Instruction O:013
I:012 I:012
I:012
11
15
O:013
01
OUTPUT ENERGIZE instruction - TRUE Lecture – PLC Programming Basics
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Output Energize (OTE) Instruction O:013
I:012 I:012
I:012
11
15
O:013
01
OUTPUT ENERGIZE instruction - FALSE Lecture – PLC Programming Basics
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Status Bit Examples Input module
A
Bit status
Button not actuated Output
A
OFF
False A
Output ON
True Lecture – PLC Programming Basics
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Status Bit Examples Input module
A
Bit status
Button actuated Output
A
ON
True Output
A
OFF
False Lecture – PLC Programming Basics
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Ladder Rung A
B
C
Output instruction
Input conditions D
A ladder rung consists of a set of input conditions, represented by contact instructions, and an output instruction at the end of the rung, represented by the coil symbol. Lecture – PLC Programming Basics
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Ladder Rung A
B
C
Output instruction
Input conditions D
For an output to be activated or energized, at least one left-to-right path of contacts most be closed. A complete path is referred to as having logic continuity. When logic exists the rung condition is said to be TRUE. Lecture – PLC Programming Basics
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Rung Continuity Bit in memory
Bit in memory
1
1 LS_1
SOL_5
The Examine If Closed instruction is TRUE making the rung TRUE Lecture – PLC Programming Basics
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Rung Continuity Bit in memory
Bit in memory
0
0 LS_1
SOL_5
The Examine If Closed instruction is FALSE making the rung False Lecture – PLC Programming Basics
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Allen-Bradley SLC-500 Controller Addressing Output image table file 0
Address output terminal O0:4/6
O:0:4/6 Bit address Address input terminal I1:3/12
Input image table file 1
Energized output
I:3/12 Bit address Closed input
I1:3
O:0:4
User-programmed rung
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Structure of A 16-Bit Word Bit
OFF ON
Word (16 bits)
0 1 15 14 13 12 11 10 9
8
7
6
5
4
3
2
1
0
I/O Connection Diagram L1
L2
L1
L2
PB1
SOL 1 O:2/3
I:4/5
PL 1 LS1
O:3/6
I:4/6
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R
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Parallel Input Branch Instructions A
C
B
Lecture – PLC Programming Basics
Branch instructions are used to create parallel paths of input condition instructions. If at least one of these parallel branches forms a true logic path, the logic is enabled.
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Parallel Output Branching A
C D
B
E On most PLC models, branches can be established at both the input and output portion of the rung. With output branching, you can program parallel outputs on a rung to allow a true logic path to control multiple outputs. Lecture – PLC Programming Basics
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Nested Input and Output Branches
Input and output branches can be nested to avoid redundant instructions and to speed up the processor scan time. A nested branch starts or ends within another branch. Lecture – PLC Programming Basics
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Nested Contact Program A
B
C
Y
Nested contact
D
On some PLC models, the programming of a nested branch circuit cannot be done directly.
E A
B
C
D
Y
C
Contact instruction C repeated
Reprogrammed to obtain the required logic.
E Lecture – PLC Programming Basics
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PLC Matrix Limitation Diagram Max series contacts
No. outputs per rung and location of the output in the rung
Max parallel lines
There may be limitations to the number of series contacts instructions, number of parallel lines, and the number of outputs and their location on the rung. Lecture – PLC Programming Basics
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Programming of Vertical Contacts A
D
Y
Original program
C E D
B A B
C
A
C
Y
D E
B
Y = (AD) + (BCD) + (BE) + (ACE)
Reprogrammed to obtain the required logic
E
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Programming for Different Scan Patterns B
A
D
C
Y
Original program
E
F
Y = (ABC) + (ADE) + (FE) + (FDBC)
A
B
C
A
D
E
F
E
F
D
Y
Reprogrammed to obtain the required logic B C
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Internal Control Relay The internal output operates just as any other output that is controlled by programmed logic; however, the output is used strictly for internal purposes. The internal output does not directly control an output device. The advantage of using internal outputs is that there are many situations where an output instruction is required in a program, but no physical connection to a field device is needed. Their use in this type of instance can minimize output card requirements.
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Extending the Number of Series Contacts Using an Internal Control Relay Internal relay coil Rung 1
Rung 2 Internal relay contact
Lecture – PLC Programming Basics
Discrete output (requires one physical connection on the output module)
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Programming The XIC Instruction PB1
PB2
Hardwired Circuit PL
PB1
PB2
PL
User program providing the same results
Note that both pushbuttons are represented by the XIC symbol. This is because the normal state of an input (NO or NC) does not matter! What does matter is that if contacts need to close to energize the output, then the XIC instruction is used. Since both PB1 and PB2 must close to energize the PL, the XIC instruction is used for both.
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Programming The XIO Instruction PB1
PB1 CR
PL
CR1 PL
User program providing the same results
Hardwired Circuit When the pushbutton is open in the hardwired circuit, relay coil CR is deenergized and contacts CR1 close to switch the PL on. When the pushbutton is closed, relay coil CR is energized and contacts CR1 open to switch the PL off. The pushbutton is represented in the user program by an XIO instruction. This is because the rung must be true when the external pushbutton is open, and false when the pushbutton is closed.
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Operation of The XIC and XIO Instructions Summary of status conditions The status of the instruction is: XIC Examine If Closed
XIO Examine If Open
Logic 0
False
True
False
Logic 1
True
False
True
If the data table bit is:
Lecture – PLC Programming Basics
MME 486 – Fall 2006
OTE Output Energize
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Operation of The XIC and XIO Instructions State of the output as determined by the changing state of the inputs in the rung Inputs
Time
XIC
XIO
OTE
0
0
0
Goes true
1
0
1
False
Goes false
1
1
0
False
Remains false
0
1
0
XIC
XIO
t1 (initial)
False
True
t2
True
True
t3
True False
t4
Input instructions XIC
XIO
Bit status
Output OTE False
Output instruction OTE
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Entering the Ladder Diagram A personal computer is most often used to enter the ladder diagram.
The computer is adapted to the particular PLC model using the relevant programmable controller software.
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RSLogix Main Screen Different screens, toolbars and windows dialog boxes are used to navigate through the Windows environment
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Bit Instructions Tool Bar
To place an instruction on a rung, click its icon on the toolbar and simply drag the instruction straight off the toolbar onto the rung of the ladder.
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Select Processor Type Screen The programming software needs to know what processor is being used in conjunction with the program. 1747-L40E
You simply scroll down the list until you find the processor you are using and select it.
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I/O Configuration Screen The I/O screen lets you click or drag-and-drop a module from an all inclusive list to assign it to a slot in your configuration.
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Data File Screen Data file screens contain data that is used in conjunction with ladder program instructions. These include: Input Output Timer Counter Integer Bit
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Monitoring a Ladder Logic Program Operation of the logic is apparent from the highlighting of rungs of the various instructions on screen, which identifies the logic state in real time and has logic continuity.
Highlighted rungs indicate the instruction is true Lecture – PLC Programming Basics
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Modes of Operation A processor has basically two modes of operation: the program mode or some variation of the run mode. Program Mode – may be used to Ø enter a new program Ø edit or update an existing program Ø upload files Ø download files Ø document programs Ø change software configurations When the PLC is switched into the program mode, all outputs from the PLC are forced off regardless of their rung logic status, and the ladder I/O scan sequence is halted.
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Variations of the Run Mode Run Mode – is used to execute the user program. Input devices are monitored and output devices are energized accordingly. Test Mode – is used to operate, or monitor, the user program without energizing any outputs.
Remote Mode – allows the PLC to be remotely changed between program and run mode by a personnel computer connected to the PLC processor.
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